Supercontinuum light source

ABSTRACT

A supercontinuum light source can comprise a seed laser arranged to provide seed pulses with a pulse frequency F seed ; a pulse frequency multiplier (PFM) arranged to multiply the seed pulses by converting pulses having the pulse frequency F seed  to pump pulses with a pulse frequency F pump , where F pump  is larger than F seed ; and a non-linear element arranged to receive said pump pulses and convert said pump pulses to pulses of supercontinuum light. The PFM can further comprise a splitter for splitting pulses into first and second sub beams each having the same pulse frequency, where the PFM is configured such that the sub beams experience different delays; and a combiner for combining said first and second sub beams into a beam having the pulse frequency that is greater than said same pulse frequency. The splitter can have an uneven splitter ratio. The disclosure includes other embodiments as well.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 14/404,748,which was filed in the U.S. Patent & Trademark Office on 1 Dec. 2014 nowU.S. Pat. No. 9,504,374 B2, and which is a national stage application ofPCT/DK2013/050167, which was filed on 30 May 2013, and which claims thepriority of U.S. provisional application Ser. No. 61/659,222, which wasfiled on 13 Jun. 2012, and which claims the benefit of Danish Pat. App.No. PA 2012 00378, which was filed 1 Jun. 2012. The foregoingapplications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to supercontinuum (SC) light sources as tomeasurement systems using supercontinuum light sources. Also disclosedare supercontinuum light sources comprising an intermediatesupercontinuum light source and a single mode coupling unit, where thesupercontinuum light source is suitable for use in a measurement system,for example in a system where a sample to be measured or in other wayanalyzed is illuminated by light originating from such a supercontinuumlight source, where the measurement system is arranged to allowdetection of light from the sample. The invention also relates to asystem suitable for measuring at least one parameter of an object, saidsystem comprising the supercontinuum light source as well as a method ofmeasuring at least one parameter of an object of the measurement system.

BACKGROUND INFORMATION

Optical measurement systems exist in many variations. Common to thesesystems is that a beam of light is directed to the sample and light iscaptured from the sample. The captured light may be light reflected fromthe sample, transmitted through the sample and/or light emitted from thesample in response to the incoming beam such as fluorescence.

Octave bandwidth supercontinuum (SC) has been successfully generateddirectly through non-linear fibers, such as microstructured fibers,tapered standard fibers, and tapered microstructured fibers by pumpingthe fiber with pulsed lasers (often in a MOPA configuration) as input.Such a spectrally broad continuum source is potentially useful in manymeasurement systems, such as optical coherence tomography (OCT), opticalfrequency metrology, fluorescent microscopy, coherent anti-Stokes Ramanscattering (CARS) microscopy, and two-photon fluorescence microscopy.Unfortunately, for those experiments, the large amplitude fluctuationsof conventional continuum sources limit accuracy and/or sensitivity.Previous studies of SC generation have shown that the SC generationprocess is very sensitive to quantum noise, technical noise, andspecific parameters such as the input wavelength, time duration, andchirp of the input laser pulses. A light source derived from a stablecontinuum would generally improve the usefulness of SC sources.

Continuum generation in conventional holey, photonic crystal, or taperedsingle-mode long fibers is complex and can contain significantsub-structures in the time and frequency domains leading to undesirableand unevenly distributed noise and instability for different wavelengthregions. Usually, the amplitude of the continuum shows largefluctuations with significant excess white-noise background, which canbe revealed with a fast detector and RF spectrum analyzer (RFSA)measurement.

A common approach to wavelength conversion is to generate asupercontinuum, then spectrally slice off part of the continuum and usethis slice as the light source for the microscopy setup. However, theselected continuum likely contains large amplitude fluctuations (noise),which may not be suitable for some applications.

In U.S. Pat. No. 7,403,688, noise from the SC source is reduced bytapering the non-linear fiber and using a femto-second pulse sourcewhich gives rise to so-called soliton fission. The abstract of thispatent states: “The longitudinal variation of the phase-matchingconditions for Cherenkov radiation (CR) and four-wave mixing (FWM)introduced by DMM allow the generation of low-noise supercontinuum.”Tapering requires either a post processing technique or variation ofdiameter of the fiber during production which may complicate theproduction of the SC light source, and the small cross section of ataper may limit the amount of light which can be safely transmitted.Furthermore, femto-second pump sources are often relatively complex andexpensive.

In US2011/0116282 a light source apparatus having a base structurecapable of generating SC light and further having a structure thatenables the shaping of the spectral waveform of the SC light, poweradjustment of the SC light, or adjustment of the frequency of repetitionof the pulse train that contains the SC light is described. The lightsource apparatus of US2011/0116282 comprises a SC fiber pumped atwavelengths at about 1550 nm and the frequency of repetition of a SCoptical pulse train from the light source lies between 1 MHz or more,but at 100 MHz or less. Throughout US2011/0116282, noise is onlydiscussed in relation to single pulses, and it is described that thenoise characteristic of the pulse light P1 is not influenced. Inrelation to the noise characteristic of the SC optical pulse train P2,it is mentioned that low noise detection is possible throughsynchronization with an optical detector disposed outside the lightsource apparatus. Noise spectra from SC light sources using differentpump wavelengths differ, and thus noise suppression may differ.US2011/0116282 refers to femtosecond pulse trains P1. Such pump sourcesare often relatively complex and expensive.

SUMMARY OF DISCLOSURE

In view of the foregoing, an object of the present invention is toprovide a low noise supercontinuum light source and, advantageously, asupercontinuum light source with a reduced impact of noise in thegenerated supercontinuum (SC). The supercontinuum light source isadvantageously suitable for use in an optical measurement system.

In an embodiment, the invention relates to a system suitable formeasuring at least one parameter of an object, said system comprisingthe supercontinuum light source. The invention also relates to providing[[e]] a method of measuring using the system.

These and other objects have been solved by the invention or embodimentsthereof as defined in the claims and as described herein below.

It has been found that the invention and embodiments thereof have anumber of additional advantages which will be clear to the skilledperson from the following description.

The supercontinuum light source of the invention comprises a lightsource output, an intermediate supercontinuum light source, and a singlemode coupling unit, wherein said intermediate supercontinuum lightsource comprises

-   -   a. a seed laser arranged to provide seed pulses with a pulse        frequency F_(seed);    -   b. a pulse frequency multiplier (PFM) arranged to multiply the        seed pulses and convert F_(seed) to pump pulses with a pulse        frequency F_(pump), where F_(pump) is larger than F_(seed);    -   c. a non-linear element arranged to receive said pump pulses and        convert said pump pulses to a supercontinuum light provided as        an output of said non-linear element and having a supercontinuum        spectrum spanning from about λ₁ to about λ₂ where λ₁-λ₂>about        500 nm.

The output from the non-linear element is coupled to the single modecoupling unit to provide an output from the single mode coupling unit,and the light source output comprises the output from the single modecoupling unit. The single mode coupling unit is arranged to dampen andshape said supercontinuum spectrum from said non-linear element.Preferably F_(pump) is at least about 100 MHz, such as at least about150 MHz, such as at least about 200 MHz, such as at least about 300 MHz,such as at least about 400 MHz, such as at least about 500 MHz, such asat least about 600 MHz, such as at least about 700 MHz, such as at leastabout 800 MHz, such as at least about 1 GHz.

In a preferred embodiment of the frequency multiplier, said single modecoupling unit is arranged to receive said supercontinuum light andspectrally shape it so that the output spectrum from said single modecoupling unit is spanning from λ₃ to λ₄, where λ₃-λ₄>0, λ₃≤λ₁ and λ₄≥λ₂,and wherein the spectrally shaped output spectrum output from the singlemode coupling unit is different from the spectrum in the wavelengthrange from λ₃ to λ₄ from the intermediate supercontinuum source.

It has been found that the supercontinuum light source of the presentinvention has a low-noise resulting in a highly improved supercontinuumlight source in particular for applications where low-noise isbeneficial. The term “low-noise” is herein taken to mean average noisesignificantly lower than would otherwise have been possible with priorart white light SC source operating at comparable power level of outputpower in the spectral range, such as significantly lower than wouldotherwise have been possible with a prior art supercontinuum lightsource operating at comparable power level of output power and above thesoliton fission regime e.g. when the source is applied in themeasurement system.

The seed laser of the intermediate supercontinuum light source can, forexample, be a mode-locked fiber laser, preferably mode-locked via aSESAM, preferably the gain medium of said fiber laser is selected froman [[Yt]]Yb-doped fiber, an Er-doped fiber and an Er/Yb-doped fiber.

In an embodiment, the wavelength range “λ₃-λ₄” is larger than about 100nm, such as larger than about 200 nm, such as larger than about 300 nmor such as larger than about 500 nm. In an embodiment, the wavelength λ₄is smaller than about 1000 nm, such as smaller than about 900 nm, suchas smaller than about 800 nm, such as smaller than about 700 nm, or suchas such as smaller than about 600 nm. In an embodiment, λ₃ is largerthan about 1070 nm, such as larger than about 1100 nm, such as largerthan about 1200 nm, or such as larger than about 1300 nm. In anembodiment, the single mode coupling unit comprises one or more of thefollowing: a prism, a low-pass optical filter, a high-pass opticalfilter, a bandpass optical filter, and a single mode fiber.Advantageously, the single mode coupling unit is arranged to shape thespectrum from the intermediate supercontinuum light source into aGaussian spectrum, a double peak spectrum or a flat top spectrum.

In an embodiment, the dampening of the supercontinuum spectrum in saidsingle mode coupling unit is given by an optical power dampening factory, said optical power dampening factor y being a measure of the opticalpower dampening within the wavelength range from λ₄ to λ₃, wherein saidoptical power dampening factor y is larger than about 2, such as largerthan about 3, such as larger than about 4, such as larger than about 6,such as larger than about 8, such as larger than about 10.

In an embodiment, the single mode coupling unit comprises at least oneof the following in order to carry out said dampening: i) misalignmentor mismatch of the output from the non-linear element to the single modecoupling unit; ii) splice loss at the input to and/or output from thesingle mode coupling unit; and iii) a broadband attenuation filter, suchas a neutral density filter or a broadband beam splitter. In anembodiment, the single mode coupling unit comprises an input forcoupling to the non-linear element; a dichroic element at the input ofthe single mode coupling unit, said dichroic element being arranged totransmit wavelengths below a threshold wavelength λ₅, wherein λ₅>λ₃; atleast one of the following: a prism, a low-pass optical filter, ahigh-pass optical filter or a bandpass optical filter; and a single modefiber, the output of which is the output from the single mode couplingunit. Advantageously, the dichroic element is a single-mode fiber, saidsingle-mode fiber being a step index fiber or a micro-structured fibercomprising micro-structures in the form of air or low-index glassmaterial.

In an embodiment, the total optical power at the output from said singlemode coupling unit is less than about 100 mW, such as less than about 50mW, such as less than about 30 mW, such as less than about 20 mW.

In an embodiment, the seed laser is arranged to provide seed pulses withpulse duration t_(seed), said pulse duration t_(seed) being longer thanabout 0.1 ps, such as longer than about 0.25 ps, such as longer thanabout 0.5 ps, such as longer than about 0.75 ps, such as longer thanabout 1 ps, such as longer than about 2 ps, such as longer than about 3ps, such as longer than about 5 ps, such as longer than about 10 ps,such as longer than about 20 ps, such as longer than about 50 ps, suchas longer than about 100 ps, such as longer than about 200 ps, such aslonger than about 300 ps, such as longer than about 400 ps, such aslonger than about 500 ps, such as longer than about 1 ns.

In an embodiment, the seed laser is arranged to provide seed pulses withpulse duration t_(seed), wherein said pulse duration t_(seed) is shorterthan about 1 us, such as shorter than about 500 ns, such as shorter thanabout 200 ns, such as shorter than about 100 ns, such as shorter about50 ns, such as shorter than about 20 ns, such as shorter than about 10ns, such as shorter than about 1 ns, such as shorter than about 500 ps,such as shorter than about 100 ps, such as shorter than about 50 ps,such as shorter than about 25 ps, such as shorter than about 20 ps, suchas shorter than about 15 ps, such as shorter than about 10 ps.

Advantageously, the non-linear element is an optical fiber, such as atapered and/or untapered microsctructured fiber.

In an embodiment, the intermediate supercontinuum light source comprisesa pulse compressor, such as a PBG fiber, said pulse compressor beingarranged to receive the pulses from said pulse frequency multiplier(PFM) and to output time-compressed pulses to said non-linear element.Advantageously, the intermediate supercontinuum light source is anincoherent light source.

The system is suitable for measuring at least one parameter on anobject, comprises the supercontinuum light source of the invention, andis arranged to illuminate the object to be measured with at least partof an output of said single-mode coupling unit, such as the major part,such as at least about 90%, of all of the output of said single modecoupling unit, the system further comprising a detector for detectinglight from said object.

Due to the supercontinuum light source of the invention comprising a lownoise intermediate supercontinuum light source, a very accurate opticalmeasurement system is achieved.

In an embodiment, the system comprises the object, and the object ispart of a human or animal body, such as a mammalian eye or any partthereof. Hereby, in vivo and/or in vitro measurements of parts of thehuman or animal body are possible.

Advantageously, the detector has an integration time being longer thanthe 50/F_(pump), such as longer than 100/F_(pump), such as longer than200/F_(pump), such as longer than 500/F_(pump), such as longer than1000/F_(pump), such as longer than 5000/F_(pump).

In an embodiment, the measuring system is a reflection mode measurementsystem arranged to measure light reflected from said object, such as asystem based on white light interferometry, such as Optical CoherenceTomography (OCT). Advantageously, the system is based on time domain,frequency domain or swept source OCT.

In an embodiment, the measuring system is used for diagnosis ofAge-related macular degeneration (AMD), diabetic retinopathy orglaucoma.

In an embodiment, the measuring system is used for diagnosis inconnection with treatment to correct refractive eye corrections, such ase.g. laser eye surgery to correct refractive eye conditions (LASIK). Inan embodiment, the measuring system is used for measuring the boundariesof the Bowman layer inside a human eye. The method of the invention formeasuring at least one parameter on an object to be measured comprisesproviding a supercontinuum light source of the invention; illuminatingthe object to be measured with at least part of an output of saidsingle-mode coupling unit of the supercontinuum light source of theinvention, such as all of the output of said single mode coupling unit;and detecting light from said object by a detector.

Due to the high accuracy of the optical measurement system the object isadvantageously a part of a human or animal body, such as mammalian eyeor a part thereof. Hereby, in vivo and/or in vitro measurements of partsof the human or animal body are possible.

In the following the invention will be described in relation tosilica-based non-linear fibers; however, as will be clear to the skilledperson, the invention will also include SC sources based on other typesof non-linear elements such as fibers based on other materials (such ase.g. polymers, chalcogenide and fluoride glasses), non-linear planarwaveguides and gas-filled hollow-core fibers. Relative to silica-basedfiber parameters, material and/or waveguide based parameters, such ase.g. dispersion and non-linearity, will have to be adjusted accordingly.

Typically, SC is generated by applying a pulsed pump light sourcearranged to pump a non-linear fiber, such as a non-linear fiber asdiscussed above. Non-linear processes in the non-linear element convertthe pump pulses to a supercontinuum exiting the fiber. Of particularinterest is the case where substantial pump energy is provided towavelengths in the non-linear fiber exhibiting anomalous dispersionsince this greatly extends the achievable bandwidth. In particular,supercontinuum generation based on so-called modulation instabilitywhere the pump pulse breaks up into a series of short pulses (solitons)which allow the generation of efficient and broad supercontinuumspectra, as described by Dudley et al in Rev. Mod. Phys. Vol. 78, No. 4,(2006). In the normal dispersion regime the supercontinuum generation iscaused primarily by self-phase modulation (SPM) which requires very highpeak intensity to induce significant spectral broadening (e.g. >100 nm10 dB bandwidth). Accordingly, in an embodiment, the pump pulses and thenon-linear fiber (i.e. the non-linear element) are arranged so that thesupercontinuum spectrum is generated mainly through modulationinstability (MI) induced breakup of the pump pulses, i.e. most of theinput pulse power is launched at wavelengths situated in the anomalousregime—or sufficiently close to allow initial spectral broadening viaSPM to shift a substantial part of the power into the anomalous regime.Preferably more than 50% of the generated supercontinuum spectrum isgenerated via MI and subsequent processes involving the solitonsgenerated by MI, such as more than 60%, such as more than 70%, such asmore than 80% such as more than 90%, such as more than 95%, such as100%. Any residual pump light exiting the non-linear element is notconsidered part of the generated supercontinuum. In an embodiment, thesepercentages are calculated as part of the total power of thesupercontinuum. In an embodiment, the percentage is calculated as apercentage of the bandwidth spanned by the supercontinuum.

The high nonlinearity of so called ‘Highly Nonlinear Fibers (HNLF) isgenerally a consequence of relatively small cross sections giving riseto increased peak intensity, but more importantly, the dispersion ofthese fibers is typically low and anomalous at least for part ofwavelength, and the fiber will guide e.g. at the pump wavelength. Theformer ensures long effective nonlinear interaction length because peakpower is maintained, and the latter supports soliton formation and MIbreakup. In an embodiment, soliton formation and MI induced breakup arekey mechanisms in ultra broadband light generation from nonlinearfibers. Other nonlinear processes, such as self-phase modulation,cross-phase modulation, self-steepening, Raman scattering, although notrequiring anomalous dispersion, also play a part.

The pump pulses and the non-linear element may be arranged so that thecenter wavelength of the pump pulses is preferably in the anomalousdispersion regime. Alternatively, the pump wavelength could be in thenormal dispersion regime but sufficiently close to the anomalous regimethat modest spectral broadening can transfer a substantial part of thepump energy to the anomalous regime (e.g. via SPM or Raman shifting),such as more than or equal to ZDW-150 nm, such as more than or equal toZDW-100 nm, such as more than or equal to ZDW-50 nm, such as more thanor equal to ZDW, such as more than or equal to ZDW+10 nm, such as morethan or equal to ZDW+20 nm, such as more than or equal to ZDW+30 nm,such as more than or equal to ZDW+50, such as more than or equal toZDW+100 nm, such as more than or equal to ZDW+150 nm. In an embodiment,the shape of the resulting supercontinuum spectrum can, to a greatextent, be controlled by varying the distance from the pump wavelengthto the crossing between normal and anomalous dispersion—the so-calledzero dispersion wavelength (ZDW).

The term “substantial pump energy shifted into the anomalous region” istaken to mean that more than 30% of the pulse energy enters theanomalous region before the pulse breaks up, such as more than 50%, suchas more than 60%, such as more than 70%, such as more than 80% such asmore than 90%, such as more than 95%, such as 100%.

As described by Dudley et al. in “Supercontinuum generation in photoniccrystal fiber”, Rev. Mod. Phys., Vol. 78, No. 4, (2006) pp. 1159-1162 asupercontinuum will be incoherent if modulation instability is thedominating process in the breakup of the pump pulses. An incoherentsupercontinuum can be understood as originating from noise and thereforethe temporal and spectral stability of the generated light iscompromised. According to the authors, pump pulses having a solitonorder (N) in the fiber of N<10 provides a coherent supercontinuum,whereas pump pulses having N>30 provides an incoherent supercontinuum.Values of 10≤N≤30 provide a transition between these two states, where asupercontinuum spectrum may be generated coherently or incoherentlydepending on the exact pump and fiber parameters. Here, the solitonorder is defined as (Eq. 1):

$N = \sqrt{\frac{\gamma \cdot P_{0} \cdot T_{0}^{2}}{\beta_{2}}}$

Where γ is the fiber nonlinearity, P₀ is the pulse peak power, T₀ is thepulse length and β₂ is the group velocity dispersion of the fiber at thepump wavelength. This equation therefore confirms that short pulsesreduce the solitons order providing for a more coherent supercontinuumand thus lower noise.

The coherence may be reduced dramatically (and noise increasesdramatically) when N>16. The increased value of N cause modulationinstability—which is a pulse breakup induced by quantum noise—to proceedfaster than the deterministic soliton fission process. Hence thetransition from soliton fission to MI-induced breakup marks theseparation between low noise/high coherence and high noise/lowcoherence. In “Generation of a broadband continuum with high spectralcoherence in tapered single-mode optical fibers”, Fei Lu, et al., OpticsExpress, Jan. 26, 2004, vol. 2, No. 2, pp. 347-353 (which is referencedin U.S. Pat. No. 7,403,688 and have authors corresponding to theinventors) short 50 fs pulses provide a relatively low N, and thesolitons order is further reduced by tapering providing a high spectralcoherence and low-noise. In “Super continuum generation for real timeultrahigh resolution optical coherence tomography”, Proc. of SPIE Vol.6102, 61020H, (2006) supercontinuum is generated using 95 fs pump pulsesand it is concluded that only spectra generated by pumping in the normalregime have sufficiently low noise to be applicable. As noted above,such spectra are formed by SPM, which is a deterministic process andthus allows generation of low-noise, highly coherent SC.

In an embodiment, the non-linear fiber is untapered; however, in anembodiment the present invention is combined with the noise reductioneffect obtainable via tapering. Novel types of tapered fibers suitablefor SC generation are described in the International ApplicationPCT/DK2011/050328.

However, in an embodiment, the present invention allows the applicationof incoherent or partially incoherent supercontinuum, so that, in anembodiment, the non-linear fiber and the pump pulses are arranged sothat the solitons order of said pump pulses is substantially higher thanor equal to 16, such as equal to or more than 18, such as equal to ormore than 20, such as equal to or more than 22, such as equal to or morethan 24, such as equal to or more than 26, such as equal to or more than28, such as equal to or more than 30, such as equal to or more than 40,such as equal to or more than 50, such as equal to or more than 75, suchas equal to or more than 100, such as equal to or more than 200, such asequal to or more than 300, such as equal to or more than 400, such asequal to or more than 500. Thereby the supercontinuum generation processproceeds mainly via modulation instability.

In an embodiment, the soliton order is defined when the pulse breaks upe.g. after shifting to the anomalous regime and/or after traversing atapered section of the fiber. In an embodiment, the soliton order isdefined at the entry of the pump pulse into the fiber.

Commonly, the spectral width of the generated SC depends on the peakpower of the pump pulses, so for longer pulses the peak power cannot bearbitrarily reduced in order to reduce the soliton order. Longer pulses,such as pulses in the ps-regime or ns-regime, are often preferable asthese pulses often allow a simpler pump laser design relative tofs-lasers. Accordingly, in an embodiment the invention allows theapplication of longer pulse durations such as application where thepulse duration is longer than about 0.1 ps, such as longer than about0.25 ps, such as longer than about 0.5 ps, such as longer than about0.75 ps, such as longer than about 1 ps, such as longer than about 2 ps,such as longer than about 3 ps, such as longer than about 5 ps, such aslonger than about 10 ps, such as longer than about 20 ps, such as longerthan about 50 ps, such as longer than about 100 ps, such as longer thanabout 200 ps, such as longer than about 300 ps, such as longer thanabout 400 ps, such as longer than about 500 ps, such as longer thanabout 1 ns, such as longer than about 10 ns.

On the other hand, SC generated from very long pump pulse and CW,pumping suffers from increased noise. While the present invention mayreduce sensitivity to noise, it may be preferable to also decrease thenoise via reducing pulse duration as well, so that in an embodiment theseed laser is arranged to provide seed pulses with pulse durationt_(seed), wherein said pulse duration t_(seed) is shorter than about 1μs, such as shorter than about 500 ns, such as shorter than about 200ns, such as shorter than about 100 ns, such as shorter about 50 ns, suchas shorter than about 20 ns, such as shorter than about 10 ns, such asshorter than about 1 ns, such as shorter than about 500 ps, such asshorter than about 100 ps, such as shorter than about 50 ps, such asshorter than about 25 ps, such as shorter than about 20 ps, such asshorter than about 15 ps, such as shorter than about 10 ps.

The open-ended intervals mentioned above may be combined to form closedintervals for the pulse duration, such as the pulse duration beingbetween 0.1 ps and 1 μs, such as between 0.25 ps and 100 ps, such asbetween 1 ps and 50 ps. As noted above, SC is typically generated byapplying a pulsed pump light source. In the supercontinuum light sourceof the invention, the pump pulses are provided with a repetition rate,F_(pump), which results in an amplitude modulation of the generatedsupercontinuum with the same frequency, F_(pump). On the other hand, themeasurement system of the invention typically applies a measurementtime, which is longer than 1/F_(pump) over which the measurement isintegrated so that the repetition rate is not resolved and the SCappears as CW radiation. Pulsed lasers operating in the MHz range areoften referred to as ‘quasi CW’ for that reason. However, the pulsednature of the supercontinuum reduces the effective measurement timewhere light is present. Therefore, in an embodiment the SC light sourceapplies a high repetition rate so that F_(pump) is 100 MHz or more, suchas 150 MHz or more, such as 200 MHz or more, such as 300 MHz or more,such as 400 MHz or more, such as 500 MHz or more, such as 600 MHz ormore, such as 700 MHz or more, such as 800 MHz or more, such as 1 GHz ormore.

As will be further discussed below, a pump laser system typicallyconsists of a master laser oscillator also referred to as a seed laserfollowed by one or more optional optical amplifiers which boost thepower level of the pulses from the seed laser, i.e. the pump laser maycomprise a MOPA configuration. Depending on the type of seed laser, itmay not be practical or possible to provide such high repetition rates.In an embodiment, the pump laser (also referred to as the pump lasersystem) comprises a seed laser arranged to provide seed pulses withpulse frequency, F_(seed), lower than F_(pump), and one or more pulsefrequency multipliers (PFM) arranged to convert F_(seed) to F_(pump).

Preferably the pulse frequency multiplier of the supercontinuum lightsource of the invention comprises a splitter dividing at least one beamof the seed pulses into a number of sub beams and a first combinerarranged to recombine at least some of the sub beams. Preferably, thepulse frequency multiplier further comprises an adjustable attenuatorarranged to adjust at least one of the sub beams.

A beam herein means a train of pulses.

The splitter may be any kind of splitter. Such splitters are well knownin the art.

In an embodiment, the pulse frequency multiplier comprises theadjustable attenuator arranged to receive at least one sub beam.Preferably, the adjustable attenuator is arranged to receive at leastone sub beam with a power above average sub beam power, optionally thepulse frequency multiplier comprises a plurality of adjustableattenuators, preferably each arranged to receive at least one sub beamhaving pulses within a selected peak power range. Advantageously forsignificantly reducing noise, the adjustable attenuator is arranged toreceive and adjust the pulses of the at least one sub beam to a peakpower value corresponding to the peak power value of the pulses of atleast one other sub beam, such that the pulses of the sub beams combinedin the first combiner have substantially identical peak power value.

In an embodiment, the pulse frequency multiplier is configured to timedelay at least one of the sub beams. The time delay can e.g. be providedby arranging first a path from the splitter to the combiner of onesub-beam to be shorter than a second path from the splitter to thecombiner of a second sub-beam. Preferably, the pulse frequencymultiplier is configured to time delay the at least one sub beam suchthat the pulses of the sub beams recombined in the first combiner arespaced, preferably with a substantially even spacing.

FIG. 1a illustrates the configuration of a preferred intermediatesupercontinuum light source 100 being comprised in the supercontinuumlight source according to the invention. The master oscillator (or seedlaser) provides an output along the beam path 106. The components arepreferably fiber coupled but may also be coupled via free-space optics.The intermediate supercontinuum light source 100 comprises two poweramplifiers (PA1 and PA2) 102 and 104. As noted above, these amplifiersare optional, but provide increase in pulse energy and peak powerrelative to the output from the seed laser 101. The seed laser 101, PA1102 and PA4 104 are each pumped by diode lasers; however, other pumpsources such as an electrical power source could alternatively be used.An optional regulator 105 is included to illustrate that theintermediate supercontinuum light source may comprise a feedback system.A feedback loop is in this embodiment formed by the photo diode 109measuring a part of the output 108 and providing one or more parametersrelated to the beam to a decision point 114 which regulates the input tothe non-linear element 107. Such a regulator may, as an example, beformed by the adjustable attenuator arranged to adjust the optical powerentering the non-linear element 107. Co-pending U.S. patent applicationSer. No. 12/865,503 (which is hereby incorporated) discuss variousembodiments of feedback loops in SC light sources (see e.g. FIG. 1 andthe claims), such as alternative placements of the regulator 105 and thephotodiode 109, various embodiments of the regulator, beam collection tothe photo diode, and the possibility of applying a feedback response toone or more of the pump sources 110-112.

The PFM 103 may be placed before the first amplifier, between theamplifiers and before the non-linear fiber. In an embodiment, the pulsetrain saturates the amplifier (PA1 and/or PA2) so that the peak power ofthe pulses out of the amplifier is constant, regardless of their inputpower. In FIG. 1, the PFM is placed between two power amplifiers (inthis case PA1 and PA2). This may be preferable because in most cases thePFM will redistribute the optical power from the seed laser to a highernumber of pulses and may have a significant insertion loss so that ifthe output pulses of the seed laser are relatively weak, the PFM mayproduce a pulse train with too low average power for it to beefficiently amplified in a subsequent amplifier. For this reason, it isin an embodiment preferable to place the PFM after one or moreamplifiers, such as between two amplifiers. On the other hand, placingthe PFM after one or more amplifiers will increase the nominal powerlost due to such insertion loss. For this reason, it is in an embodimentpreferable to place the PFM before one or more amplifiers, such asbetween two amplifiers. This may also have the effect of reducing thepeak power of the pulses passing one or more power amplifier (or othercomponents in the system), which in turn may have one or more benefitssuch as reduced non-linearity in the pump laser system. Suchnon-linearity often has the effect of broadening the pulses, which mayresult in a reduced peak power level into the non-linear element, whichin turn may reduce the spectral width of the generated supercontinuum.In an embodiment, multiple PFMs are applied such as multiple PFMsseparated by an optical component such as an optical amplifier,attenuator, compressor or filter.

In an embodiment, there is an upper limit to the allowable averageoptical power illuminating the object to be measured (also referred toas the sample). Examples of such applications include applications wherethe object is sensitive to optical power (average power and/or peakpower) over a certain threshold—that would be the case for mostbiological samples—and in specific for parts of a mammalian eye, such asthe retina. An example of application where the object is a mammalianeye ophthalmic includes imaging using OCT to image the retina or thecornea and Multi-photon fluorescence microscopy of the retina or thecornea.

In an embodiment, the output of the SC light source or a subsectionthereof must conform to one or more of the laser standards Class 1, 1M,2, 2M, 3R, 3B. In an embodiment, the power of output of the SC source isreduced so that the SC source itself may have a higher output AEL(acceptable emission level) than the above cited classes such as 100%more or higher, such as 200% more or higher, such as 400% more orhigher, such as 800% more or higher.

In an embodiment, a relatively low noise induced due to pulse length isdesirable so that pulse duration in the range of 0.5 ps-30 ps ispreferable, such as pulse duration in the range of 1 ps-20 ps ispreferable such as 2 ps-20 ps. In an embodiment, an increased averageoptical power relative to present systems is not desirable so that theaverage optical power from the SC source is less than 5 Watt output perps pulse duration, such as less than 3 Watt output per ps pulseduration, such as less than 2 watt per ps pulse duration, such as lessthan 1 watt per ps pulse duration. In one embodiment a total averageoptical power in the visible range (400 nm-850 nm) is arranged to beless than 100 mW, such as less than 50 mW, such as less than 30 mW, suchas less than 20 mW. As noted elsewhere, the reduction of average powerafter output from the SC source is often undesirably complex orimpossible as the optical components required to reduce the power alterthe spectrum.

As previously noted above, in an embodiment, the spectral width of thegenerated SC depends on the peak power of the pulses at least to acertain saturation level where further increase of peak power does notincrease the spectral width. Also, the conversion efficiency from pumplight to SC light depends on peak power, which means that for a fixedpulse width the peak power (and corresponding average power) cannot justbe reduced. Below a certain value, the desired spectral width of thegenerated spectrum will be compromised and eventually, poor conversionefficiency will result in too much unconverted pump light coming throughthe fiber—which could compromise the sample under observation.Therefore, in an embodiment, a consequence of a minimum peak power, theinsertion of a PFM cause an increase in average optical output powerrelative to the configuration where the PFM is omitted. This occursbecause the repetition rate of the pump pulses is increased while thepeak power and pulse duration are constant. In an embodiment, theoptical power is reduced by adjusting the pump energy provided to thelast power amplifier before the non-linear element but as mentioned,this may compromise the resulting spectral width.

In an embodiment, a reduction of average optical power may be performedby introducing a dampening after the non-linear element, such asattenuation or splitting part of the beam away from the beam path.Application requiring a tunable fraction of the generated spectrumdirected to the sample may apply an AOTF to perform such function. In anembodiment the AOTF may be controlled so as to reduce the amount ofaverage optical power direct to the sample. For applications requiringbroadband illumination, as e.g. in an OCT imaging system, it may be morechallenging to apply optical components to the beam without disruptingthe shape of the spectrum and/or to damage said optical element. In anembodiment, the pump laser system comprises a pulse compressor, such asa PBG fiber (hollow or solid core), arranged to compress the pump pulsesand thus increase peak power. This use of PBG fiber was discussed in thePCT Application WO2005041367. By increasing the peak power of theindividual pulse, the use of a pulse compressor will in an embodimentallow the use of a lower average optical power while maintaining thespectral characteristics of the generated spectrum.

In principle, a PFM of the intermediate supercontinuum light source ofthe invention may be any optical component suitable for receiving atrain of pulses at a repetition rate, and convert this input to a trainof pulses with a higher repetition rate. In an embodiment, the input andoutput pulses have substantially the same pulse duration and wavelength.In an embodiment, the PFM functions by splitting the pulse train at theinput into a plurality of sub pulse trains which each experience adifferent delay (optical path length) before being recombined. Therelative delay(s) cause(s) a temporal shift of the sub pulse trains whenrecombined, so that the combined pulse train comprises a higher numberof pulses than the input. For example, the input pulse train may besplit into two sub pulse trains (or sub beams) where one pulse train isdelayed in relation to the other. The repetition rate of the combinedtrain will then be doubled. Preferably, the relative shift between thebeams corresponds to half the spacing between two pulses in the inputpulse train. In an embodiment, this principle is expanded so that theinput beam is initially split into more than two sub beams, such as two,three or four sub beams, each delayed in relation to one another andrecombined. It is well-known that optical splitters (or combiners)function in a symmetrical manner. The combination of several opticalbeams result in the same amount of output beams. In an embodiment, onlya single output is used/available, whereas the optical power designatedfor the other outputs is lost in the optical system. In an embodiment,it is therefore advantageous to cascade couplers/splitters such asdiscussed in relation to FIG. 2b below.

In an embodiment, the invention relates to a PFM comprising a splitterdividing a beam into sub beams, an optional adjustable attenuatorarranged to receive a sub beam, and a first combiner arranged to combinethe sub beams. In this way the adjustable attenuator may be adjusted tocompensate for production variations in the splitter and/or combiner aswell as coupling variations, so that a resulting pulse train of pulseswith even peak power may be produced. In an embodiment, the preciseadjustment of the peak amplitude is not required, and a substantialdifference between the peak power of the recombined sub beams isacceptable.

In an embodiment, one or more splitters and combiners are arranged tohave an uneven split ratio (such as

$\frac{x}{1 - x}$where x is a percentage e.g. 45/55, 40/60, 35/65 or 30/70), and saidattenuator is arranged to receive the most powerful sub beam (or thelarger contributing sub beam in the combination of the beam), which mayensure that the more powerful sub beam can be attenuated to provide anequal power level as the other sub beam when the sub beams are combined.Thereby the noise is significantly reduced compared with situationswhere the sub-beam had different power levels.

In an embodiment, the PFM comprises multiple attenuators each arrangedto receive a separate sub beam. In an embodiment, the splitter splitsthe beam into two sub beams. In an embodiment, the splitter splits thebeam into more than two sub beam, such as 3 or more, such as 4 or more,such as 5 or more, such 6 or more, such as 7 or more, such as 8 or more.In an embodiment, the first combiner further acts as splitter splittingthe combined beam into secondary sub beams followed by a secondcombiner. In an embodiment, the PFM comprises an adjustable attenuatorarranged to receive one of said secondary sub beams. This attenuator maybe applied to adjust for variations in the first combiner and secondcombiner as well as coupling loses and other variations. In anembodiment, the second combiner is arranged to have an uneven splitratio (and thus also an uneven combination of the incoming beams), andthe output from said adjustable attenuator is arranged to provide thelarger fraction to the output. Again, this may ensure that a pulse trainwith even power between the pulses can be provided by the PFM.

In an embodiment, the PFM is formed by free-space optics such as bulkbeam splitters. In an embodiment, the PFM is formed by fiber opticsplitters and/or couplers, which are often preferable in relation tocost and robustness of the system.

DETAILS OF THE DISCLOSURE

FIG. 1a shows a schematic intermediate supercontinuum light sourcesuitable for the present invention.

FIG. 1b shows an example of a supercontinuum spectrum (10) spanning fromλ₂ being about 460 nm to λ₁ being about 2400 nm.

FIGS. 2a and 2b show examples of pulse frequency modulators (PFM) of anintermediate supercontinuum light source according to the presentinvention.

FIG. 3a shows measurement setup suitable for measuring intensity noisein the spectrum of a SC light source, such as that of FIG. 1.

FIG. 3b shows an example of a supercontinuum spectrum output from theintermediate supercontinuum light source 100, as well as an example ofthe spectrum output from the single mode coupling unit 300,respectively.

FIGS. 3c, 3d and 3e show exemplified spectra output from the single modecoupling unit.

FIGS. 4a and 4b show the average intensity noise of an intermediatesupercontinuum light source after and prior to compensation forspectrometer noise.

FIG. 5 shows an optical measurement system exemplified as an OCT systemutilizing a SC light source as light source.

FIG. 6 shows an example of a single mode coupling unit comprising adichroic element being a dichroic mirror, a dispersive element being aprism and a single mode fiber arranged to shape the spectrum.

FIG. 7 shows an example of a single mode coupling unit comprising adichroic element being a single mode fiber, a dampening and/or shapingoptical element and a second single mode fiber.

FIG. 8a-8c show three examples of how to dampen optical power.

FIG. 1b shows an example of a supercontinuum spectrum (10) spanning fromλ₂ being about 460 nm to λ₁ being about 2400 nm. The spectrum isobtained from the product SuperK EXW-12 from NKT Photonics A/S.

FIGS. 2a and 2b show examples of pulse frequency modulators (PFM) of anintermediate supercontinuum light source according to the presentinvention FIG. 2a shows an embodiment of a PFM 200. The input beam(either free-space or via a fiber) enters the PFM at the input 201. Thesplitter 214 is exemplified as a 1×2 splitter but may be any 1×Nsplitter or even M×N splitter. For an M×N splitter multiple inputs maybe combined or alternatively only 1 input of the M available inputs isused. The first splitter 214 divides the input beam into two sub beams202 203 with a split ratio x₁/(1−x₁). As discussed above, the larger ofx₁ and (1−x₁) is in an embodiment sent to the adjustable attenuator 204.In an embodiment, the attenuator is omitted in which case it ispreferable that x₁ is about 0.5 (i.e. 50%) so variations on the peakpower of the pulse train at the output 207 may be minimized. The subbeam 202 is subjected to a delay line 205 which is preferably arrangedto delay the sub beam 202 with one half of the period between two pulsesin the input beam 201. In an embodiment the delay line is adjustable inorder to accommodate variations in the repetition rate of the inputbeam. In an embodiment, small deviations (such as e.g. less than 75%,such as less than 50%, such as less than 25%, such as less than 15%,such as less than 10%, such as less than 5%, such as less than 1%) froman even spacing of the pulses in the output beam can be tolerated sothat the delay line is fixed. The sub beams 202 203 are combined at thecombiner 206 providing an output 207. The combiner 206 has a split ratioof x₂/(1−x₂). In an embodiment, either the splitter 214 or the combiner206 is arranged to have an uneven split ratio i.e. either x₁ or x₂deviates from 50%, in this way the attenuator 204 may be adjusted so thebeams 202 and 203 contribute evenly so that a pulse at the input dividedinto two pulses is recombined to have substantially the same peak powerat the output, where “substantially” means to include what is within theordinary tolerances. The effect of the PFM is a doubling of the pulsefrequency of the input beam. The combiner 206 further has an output 208,which may or may not be a physically available and actual output.However, the output 208 is included to illustrate that the combinerintroduces an insertion loss due to the inherent symmetry of a beamsplitter/combiner so that the peak power is reduced to about 25% of thatof the input when other optical losses (such as in couplings and theattenuator) are ignored. In an embodiment, the beam at the output 208 isapplied to monitor the beam and adjust the attenuator 204.

FIG. 2b shows the PFM of FIG. 2a , but further comprising a secondcoupler 213 so that the PFM provide a quadrupling of the pulsefrequency. In principle, a quadrupling could also be obtained byexpanding the splitter 214 to a 1×4 and the coupler 206 to a 4×1coupler. However, the coupler would in this case impose an insertionloss of about 75% due to the symmetry of a beam splitter relative to theloss of about 50% imposed by the second combiner 213. The first delayline 205 is preferably adjusted to one half of the period of the input201 which results in a doubling of the pulse rate after combining in thecombiner 206 and the second delay line 212 is preferably arranged toprovide a delay half of that, i.e. one quarter of that of the input at201. The split ratio x₂/(1−x₂) is in an embodiment arranged to be evenwhere x₁ and x₃ are arranged to be uneven so that the attenuator 204 mayperform the function as described in relation to FIG. 2a and theattenuator 211 may perform a similar function of compensating forvariations in the splitting at 206 as well as the combination in thecombiner 213. It is notable that further doubling may be obtained byfurther expanding the PFM by adding couplers without increasing theinsertion loss due to symmetric splitting.

FIG. 3a shows a measurement setup where the SC light source 1000 of theinvention is arranged to illuminate a spectrometer rather than an objectto be measured. FIG. 3a shows that the supercontinuum light source 1000of the invention comprises an intermediate supercontinuum light source100 and a single mode coupling unit 300. The output from the SC lightsource 1000 is the output from single mode coupling unit 300. The outputof the intermediate SC light source 100 is the output from thenon-linear element 107 (not shown in FIG. 3a ). This output from theintermediate SC light source 100 is coupled to the input to thesingle-mode coupling unit 300. The output of the intermediate SC lightsource 100 is at least about the output from the non-linear element (107in FIG. 1a ) of the intermediate supercontinuum light source (100 inFIG. 1a ; not shown in FIG. 3a ). The single mode coupling unit 300comprises an adaptation in the form of dampening and/or shaping thespectrum according to the requirements of the application. In oneembodiment the SM coupling unit 300 comprises one of the embodiments ofco-pending PCT application PCT/DK2011/050475 (hereby incorporated), seein particular the embodiments relating to FIGS. 5a , 6, 7, 8-10, 13-15,and 17-19 as well as their variations as well as any one of the itemsand/or claims.

FIG. 3b shows an example of a supercontinuum spectrum output from theintermediate supercontinuum light source 100 (spectrum 10), as well asan example of the spectrum output from the single mode coupling unit 300(spectrum 12), respectively. In this example, the spectrum after thesingle mode coupling unit has a Gaussian distribution and is spanningfrom λ₄ being about 650 nm to λ₃ being about 950 nm. FIG. 3b thus showsthat the spectral shape after the single mode coupling unit is differentfrom the spectral shape in the same wavelength range from theintermediate supercontinuum source.

FIGS. 3c, 3d and 3e shows examples of the spectrum output from thesingle mode coupling unit 300, the spectral shapes being a Gaussian(FIG. 3c ), a flat top (FIG. 3d ) and a double peak distribution (FIG.3e ), respectively. A double peak distribution might be advantageous ifthe output from the light source is to be sent through an opticalelement with a Gaussian like transfer function (as e.g. an optical lens)prior to illuminating the object and it is advantageous to illuminatethe object with a flat top distribution.

In one embodiment, the spectral shape after the single mode couplingunit is different from the spectrum in the same wavelength range fromthe intermediate supercontinuum source, such as a Gaussian, flat top ora double peak distribution. FIGS. 4a and 4b shows measurement resultfrom a setup according to FIG. 3a . The intermediate SC light source wasdesigned according to FIG. 1.

FIG. 4a shows the average intensity noise of an intermediatesupercontinuum light source 100 (see FIG. 1) measured between 790-870 nmusing a Wasatch Cobra UD spectrometer (310) with a Basler SprintSPL4096-70 km camera as a function of power of the supercontinuum lightsource between 400 and 850 nm. FIG. 4a shows the average intensity noiseafter compensation for the spectrometer noise, whilst FIG. 4b shows theaverage intensity noise prior to compensation for the spectrometernoise. FIG. 4a contains measurements for three different pump pulsesfrequencies (F_(pump)) being 80 MHz (curve 401) 160 MHz (curve 402) and320 MHz (curve 403). It is seen that the noise decreases when the pumppulse frequency increases. The intensity noise is compensated for thenoise added by the spectrometer.

FIG. 4b shows the intensity noise data from FIG. 4a , prior tocompensation for the noise from the spectrometer. FIG. 4b containsmeasurements for three different pump pulses frequencies (F_(pump))being 80 MHz (curve 411), 160 MHz (curve 412) and 320 MHz (curve 413).Again, it is seen that the noise decreases when the pump pulse frequencyincreases.

The MO 101 is a mode-locked Yb-fiber laser with an output having acenter wavelength at about 1060 nm and pulse duration around 6 ps. Thelaser is passively mode-locked via a SESAM and provides pulses with arepetition rate of 80 MHz. This laser type is well-suited for seedingbecause the all-fiber design provides a laser which is robust andrelatively simple to produce relative to a bulk-optical setup. Themaximum repetition rate is determined by how short the cavity can bemade and the response properties of the SESAM. In practice theselimitations often impose a practical upper limit to the repetition rateof about 100 MHz. In an embodiment, other gain media may be applied toprovide other output wavelengths, and the pulse duration and repetitionrate may also be altered within the limits discussed elsewhere.

In an embodiment, the seed laser is a fiber laser, such as a mode-lockedfiber laser, such as mode-locked via a SESAM. The gain medium may beformed by any suitable laser gains medium such e.g. Yb-doped fiber, anEr-doped fiber and an Er/Yb-doped fiber. The seed laser may e.g. be alinear cavity laser or a ring laser. The non-linear medium 107 is amicrostructured PCF fiber formed by a silica core surrounded by ahexagonal pattern of holes arranged so that the core is formed by amissing hole in the pattern. The fiber is designed so that the ZDW ofthe fiber is relatively close to the pump wavelength so that substantialpump energy provided in the anomalous regime of the fiber.

As in FIG. 1 a set of optical fiber amplifiers 102,104 are arrangedaround an optional PFM. Without a PFM the pump system pumps the fiberwith approximately 10 W, 8-10 ps at 80 MHz. By inserting a PFM accordingto FIG. 2a the repetition rate is increased to 160 MHz and by insertinga PFM according to FIG. 2b the repetition rate is quadrupled to 320 MHz.FIGS. 4a and 4b show experimental results obtained using a Wasatch CobraUD spectrometer with Basler Sprint SPL4096-70 km camera arranged tomeasure the spectral range of 790-870 nm with 4096 pixels i.e. about0.02 nm/pixel. A measurement time of 12.9 μs was applied and thefluctuation of the power measured at each pixel recorded. Long and shortmeasurement times are possible such as between 1 μs and 1 ms or higher.Often, a short measurement time is desirable, such as for Fourier-domainOCT (see FIG. 4b ) where real-time imaging is often required. In FIG. 4the average relative standard deviation per pixel in the spectral rangeof 790-870 nm is measured as a function of the visible part of thespectrum. It is observed that the standard deviation and thus theintensity noise drops significantly as the repetition rate of the pumppulses is doubled, and further when it is quadrupled for equal amount ofaverage power in the visible range. The amount of power in the visiblerange depends on how effectively the pump energy is converted to visiblelight which depends on the peak power of the pump pulses and the totalamount of pump power (average power). In FIG. 4a the estimated noisecontribution from the spectrometer has been subtracted, whereas this isincluded in FIG. 4 b.

FIG. 5 shows an optical measurement system exemplified as an OCT systemutilizing a SC source as light source. The system shown in FIG. 5 is aFourier domain OCT (FD-OCT) system according to the invention where a SClight source 1000 is applied as light source thus being suitable for anoptical measurement system according to the invention. A 2×2 50/50directional splitter/coupler (501), coupled to the light source andspectrometer (310) acting as detection on one side and a lens (502), theobject to be measured (503) and a reference reflector (504) on the otherside, forms the interferometer core of the OCT system. A line scan(depth profile of the sample) is performed by a measurement of thespectrometer where the measurement depth is determined by the spectralresolution, and the spatial resolution in the sample is determined bythe spectral width of the measurement. Often, the beam is scanned overthe object to provide 2D or 3D depth profiles of the reflectivity in thesample. OCT is an extensive field comprising a large number ofvariations of the system configuration which are all expected to benefitfrom the aspects of the present invention. The output spectrum ispreferably Gaussian so that in one embodiment the SM coupling unit isarranged to shape the spectrum from the SC light source into a Gaussianspectrum, such as the embodiments discussed in relation to FIG. 5a(single band Gaussian spectrum) and FIG. 6 (dual band Gaussian spectrum)in PCT/DK2011/050475 as well as FIG. 16 arranged to provide broadtunable spectra. In one embodiment, the SM coupling unit comprises afilter arranged to provide a Gaussian spectrum. The 50/50 coupler shouldbe arranged to handle a wide spectrum and is typically either a fusedfiber coupler or a bulk optical coupler.

FIG. 6 shows an example of a single mode coupling unit 300 comprising adichroic element being a dichroic mirror, a dispersive element being aprism, and a single mode fiber arranged to shape the spectrum. Thus,FIG. 6 shows an example of how to construct the single mode couplingunit 300. The output of the intermediate supercontinuum light source 100is directed to a dichroic element 60 and a dispersive element 61. Eitherthe mirror and/or the angular dispersive element are connected to anelectronic control 6, which enables a rotation between these twoelements. The system might optionally also include a tunable dampeningfilter 62 and/or a tunable spatial filter 63. The light is collimated bya lens system 64 and collected by a fiber 65, which thereby is shapingthe spectrum. The system might optionally include a broadband splitter66, which sends a part of the light to the output 67 and another part ofthe light to a detector system 68. Said detector system is connected tothe electronic control system 6, which again is connected to thesupercontinuum light source 100 and/or the dichroic element 500 in orderto stabilize the output power. In one embodiment, the dispersive elementis a prism. In one embodiment, the fiber 65 is a single mode fiber, suchas a step-index fiber or microstructured fiber. In one embodiment, thecollimation lens system 64 comprises multiple lenses.

FIG. 7 shows an example of a single mode coupling unit 300 comprising adichroic element being a single mode fiber 60, a dampening and/orshaping optical element 70 and a second single mode fiber 65.

In one embodiment, the first single mode fiber 60 has a high loss abovea certain threshold wavelength λ₆ and thus acts as a spectral filter. Inone embodiment, the dampening and/or shaping optical element is selectedfrom the list of a prism, an optical low-pass and optical high-pass andoptical band-pass filter, a neutral density filter.

FIG. 8a-8c show three examples of how to dampen optical power in thesupercontinuum light source of the invention.

In each of the FIGS. 8a to 8c , the supercontinuum light source isdenoted by the reference number 1000, whilst the intermediatesupercontinuum light source is denoted by the reference number 100 andthe single-mode coupling unit by the reference number 300.

In FIG. 8a , the single mode coupling unit 300 comprises a dampening andshaping unit 81, where the mode field diameter at the output of thedampening and shaping unit 81 is different from the mode field diameterof a second single mode fiber 82. FIG. 8a thus shows mode field diametermismatch at the output of the dampening and shaping unit 81 of thesingle mode coupling unit 300.

In FIG. 8b , the single mode coupling unit 300 comprises a dampening andshaping unit in the form of a shaping element 83 and a dampening element84. FIG. 8c shows an example where the dampening in the single modecoupling unit 300 is obtained by having an optical splice with largeloss 86 between the intermediate supercontinuum source 100 and the inputof the single mode coupling unit 300.

It should be emphasized that the term “comprises/comprising” when usedherein is to be interpreted as an open term, i.e. it should be taken tospecify the presence of specifically stated feature(s), such aselement(s), unit(s), integer(s), step(s) component(s) and combination(s)thereof, but does not preclude the presence or addition of one or moreother stated features.

Moreover, the term “substantially” is meant to include what is withinthe ordinary tolerances.

All features of the inventions including ranges and preferred ranges canbe combined in various ways within the scope of the invention, unlessthere are specific reasons for not combining such features.

The invention claimed is:
 1. A supercontinuum light source comprising: aseed laser arranged to provide seed pulses with a pulse frequencyF_(seed); a pulse frequency multiplier (PFM) arranged to multiply theseed pulses by converting pulses having a pulse frequency of F_(seed) topump pulses with a pulse frequency F_(pump), where F_(pump) is largerthan F_(seed); a non-linear element arranged to receive said pump pulsesand convert said pump pulses to pulses of supercontinuum light; whereinsaid PFM further comprises a splitter for splitting seed pulses intofirst and second sub beams each having a pulse frequency F_(seed), saidPFM configured such that one of said sub beams is delayed relative tothe other of said sub beams; another splitter for splitting out furthersub beams each having a pulse frequency that is greater than F_(seed)and less than F_(pump), said PFM configured such that one of saidfurther sub beams is delayed relative to the other of said further subbeams; and a combiner for combining the further sub beams into a beamhaving the pulse frequency F_(pump); and wherein said seed laser isarranged to provide seed pulses with a pulse duration t_(seed) that islonger than about 1 ps.
 2. The supercontinuum light source of claim 1wherein said seed pulses have a pulse duration t_(seed) that is shorterthan about 50 ps.
 3. The supercontinuum light source of claim 1 whereinsaid seed laser comprises a mode locked Yb laser.
 4. The supercontinuumlight source of claim 3 wherein said mode locked Yb laser comprises afiber laser that is passively mode locked via a SESAM (SemiconductorSaturable Absorber Mirror).
 5. The supercontinuum light source of claim1 wherein said PFM comprises a delay line for delaying one of the firstand second sub beams.
 6. The supercontinuum light source of claim 1wherein said PFM comprises an attenuator for attenuating a sub beam. 7.The supercontinuum light source of claim 6 wherein one of said splittershas a splitter ratio that is uneven.
 8. The supercontinuum light sourceof claim 1 wherein one of said splitters has a splitter ratio that isuneven.
 9. The supercontinuum light source of claim 1 comprising firstand second amplifiers and wherein said PFM is located between theamplifiers.
 10. The supercontinuum light source of claim 1 whereinF_(pump) is 150 MHz or more.
 11. The supercontinuum light source ofclaim 1 wherein said non-linear element comprises a microstructuredoptical fiber.
 12. The supercontinuum light source of claim 1 whereinsaid another splitter comprises a coupler that combines said sub beamsand that splits said combined sub beams into said further sub beams. 13.The supercontinuum light source of claim 1 wherein the light source isconfigured such that the pulses of supercontinuum light have a pulseduration and wherein average optical power output from thesupercontinuum lights source is less than 5 Watt output per ps pulseduration.
 14. A supercontinuum light source comprising: a seed laserarranged to provide seed pulses with a pulse frequency F_(seed); a pulsefrequency multiplier (PFM) arranged to multiply the seed pulses byconverting pulses having a pulse frequency of F_(seed) to pump pulseswith a pulse frequency F_(pump), where F_(pump) is larger than F_(seed);a non-linear element arranged to receive said pump pulses and convertsaid pump pulses to pulses of supercontinuum light; and wherein said PFMfurther comprises a splitter for splitting seed pulses into first andsecond sub beams each having the same pulse frequency, said PFMconfigured such that said sub beams experience different delays; acombiner for combining sub beams into a beam having a pulse frequencythat is greater than said same pulse frequency; and an attenuator forattenuating the first sub beam such that said combiner combines thesecond sub beam with an attenuated first sub beam into said beam havingsaid pulse frequency that is greater than the pulse frequency.
 15. Thesupercontinuum light source of claim 14 wherein said splitter has anuneven splitter ratio.
 16. The supercontinuum light source of claim 14wherein said seed laser is arranged to provide seed pulses with a pulseduration t_(seed) that is longer than about 1 ps.
 17. The supercontinuumlight source of claim 14 wherein said seed laser comprises a mode lockedYb fiber laser.
 18. The supercontinuum light source of claim 14 whereinthe pulses of supercontinuum light have a pulse duration and wherein thesupercontinuum light source is configured such that the average opticalpower output from the supercontinuum light source is less than 5 Wattsper ps pulse duration.
 19. The supercontinuum light source of claim 14wherein the supercontinuum light source is configured such that thetotal average optical power in the range 400 nm-850 nm is less than 100mW.
 20. A supercontinuum light source comprising: a seed laser arrangedto provide seed pulses with a pulse frequency F_(seed); a pulsefrequency multiplier (PFM) arranged to multiply the seed pulses byconverting pulses having a pulse frequency of F_(seed) to pump pulseswith a pulse frequency F_(pump), where F_(pump) is larger than F_(seed);a non-linear element arranged to receive said pump pulses and convertsaid pump pulses to pulses of supercontinuum light; wherein said PFMfurther comprises a splitter for splitting seed pulses into first andsecond sub beams each having a pulse frequency F_(seed), said PFMconfigured such that said sub beams experience different delays; acoupler for combining sub beams and splitting the combined sub beamsinto further sub beams each having said pulse frequency that is greaterthan F_(seed) and less than F_(pump), said PFM configured such that saidfurther sub beams experience different delays; and a combiner forcombining said further sub beams into a beam having the pulse frequencyF_(ummp).
 21. The supercontinuum light source of claim 20 wherein saidseed laser comprises a mode locked Yb laser.
 22. The supercontinuumlight source of claim 21 wherein said mode locked Yb laser comprises afiber laser that is passively mode locked via a SESAM.
 23. Asupercontinuum light source comprising: a seed laser arranged to provideseed pulses with a pulse frequency F_(seed); a pulse frequencymultiplier (PFM) arranged to multiply the seed pulses by convertingpulses having the pulse frequency F_(seed) to pump pulses with a pulsefrequency F_(pump), where F_(pump) is larger than F_(seed); a non-linearelement arranged to receive said pump pulses and convert said pumppulses to pulses of supercontinuum light; and wherein said PFM furthercomprises a splitter for splitting pulses into first and second subbeams each having the same pulse frequency, said PFM configured suchthat said sub beams experience different delays; a combiner forcombining said first and second sub beams into a beam having the pulsefrequency that is greater than said same pulse frequency; and whereinsaid splitter has a splitter ratio and wherein said splitter ratio isuneven.
 24. The supercontinuum light source of claim 23 wherein said PFMcomprises a second splitter, said second splitter having a splitterratio that is uneven.